A seismic survey represents an attempt to image or map the subsurface of the earth by sending sound energy down into the ground and recording the “echoes” that return from the rock layers below. The source of the down-going sound energy might come, for example, from explosions or seismic vibrators on land, or air guns in marine environments. During a seismic survey, the energy source is placed at various locations near the surface of the earth above a geologic structure of interest. Each time the source is activated, it generates a seismic signal that travels downward through the earth. “Echoes” of that signal are then recorded at a great many locations on the surface. Multiple source/recording combinations are then combined to create a near continuous profile of the subsurface that can extend for many miles. In a two-dimensional (2-D) seismic survey, the recording locations are generally laid out along a single line, whereas in a three dimensional (3-D) survey the recording locations are distributed across the surface in a grid pattern. In simplest terms, a 2-D seismic line can be thought of as giving a cross sectional picture (vertical slice) of the earth layers as they exist directly beneath the recording locations. A 3-D survey produces a data “cube” or volume that is, at least conceptually, a 3-D picture of the subsurface that lies beneath the survey area. In reality, though, both 2-D and 3-D surveys interrogate some volume of earth lying beneath the area covered by the survey. Finally, a 4-D (or time-lapse) survey is one that is recorded over the same area at two or more different times. Obviously, if successive images of the subsurface are compared any changes that are observed (assuming differences in the source signature, receivers, recorders, ambient noise conditions, etc., are accounted for) will be attributable to changes in the subsurface.
A seismic survey is composed of a very large number of individual seismic recordings or traces. The digital samples in seismic data traces are usually acquired at 0.002 second (2 millisecond or “ms”) intervals, although 4 millisecond and 1 millisecond sampling intervals are also common. Typical trace lengths when conventional impulsive sources are used are 5-16 seconds, which corresponds to 2500-8000 samples at a 2-millisecond interval. If a non-impulsive source is used, the extended activation time of the source needs to be accommodated for, so the trace lengths will generally be longer, or recording may be continuous. Conventionally each trace records one seismic source activation, so there is one trace for each live source location-receiver activation. In the case of continuous recording, the traces may be windowed out of the continuous data in a pre-processing step, and in this case consecutive traces may overlap in time. In some instances, multiple physical sources might be activated simultaneously but the composite source signal will be referred to as a “source” herein, whether generated by one or many physical sources.
In a typical 2-D survey, there will usually be several tens of thousands of traces, whereas in a 3-D survey the number of individual traces may run into the multiple millions of traces.
After acquisition, the seismic traces will need to be processed in order to make them ready for use in exploration. One important component of such processing is obtaining accurate estimates of the subsurface velocities proximate to the survey. Having accurate subsurface velocity estimates is useful in seismic exploration in a variety of contexts. For example, the distribution of subsurface velocities can be used directly as being indicative of the geologic structure, lithology, layer content, etc. in the subsurface. Another, and arguably more important, use for such velocities is in the processing of seismic data to image the subsurface.
In many cases, an initial rough velocity model of the subsurface is available from well logs, picks from a seismic velocity analysis, etc., which is subsequently improved based on the acquired seismic data. Historically, an important method for updating a velocity model for seismic depth imaging in areas of complex geology has been based upon ray-based reflection tomography. More recently, a method known as full-waveform inversion (FWI) has been applied to update velocity models. However, because of the non-linearity of the inverse problem, a multi-scale approach is typically used in FWI, i.e., low-frequency data are inverted first, followed by data with progressively higher frequencies.
One of the uncertainties that is inherent in applying FWI with standard seismic data is that the seismic source signature is an unknown variable which must be solved for as part of the inverse problem. In addition, neither the source nor seismic data typically contain sufficient low frequencies (e.g., less than about 4 Hz) for FWI to succeed without a good knowledge, a priori, of the subsurface velocity model. As a consequence, seismic sources have been developed or modified to provide additional seismic energy in frequency bands that are important for imaging.
Sources that might be attractive in this regard include marine and land controllable seismic sources. For purposes of the instant disclosure, the term “controllable source” will be used to refer to an acoustic seismic source that radiates sound predominantly at a single frequency at a time, whose profile of frequency versus time after the start of the sweep is controllable and continuous, and whose physical limitations impose a limit on the amplitude of its output which will normally vary with frequency. Controllable sources include, by way of example only, vibroseis sources on land, and at sea, marine resonators, etc.
Of particular interest for this disclosure are controllable seismic sources of the resonant piston marine seismic variety. Information concerning same may be found in, for example, U.S. provisional patent Ser. No. 61/290,611 and its child PCT application PCT/US2010/062329, the disclosures of which are incorporated herein by reference as if fully set out at this point.
A vibrator-type controllable seismic source typically is generally asked to perform a precisely specified sweep. Ideally, every movement of the source can be controlled so that the resulting waveform matches that which is desired. However, in contrast, a resonator-type marine seismic source changes its configuration in a prescribed fashion so as to change its resonant frequency with time along a desired trajectory, but the precise details of the sweep are not constrained. So, for example, a resonant source should ideally produce a specified frequency as a function of time in the sweep, but the phase may not be so precisely specified. If the conditions are not as expected, the sweep may deviate from the desired frequency trajectory and the resulting signal may not provide the expected frequency as a function of time, or in some cases may not even contain the required frequency content.
Thus, what is needed is a way to generate a seismic signal with a controllable source such that the resulting signal has properties that make it more suitable for imaging the subsurface.
Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for a method of seismic data acquisition that would address and solve the above-described problems.
Before proceeding to a description, however, it should be noted and remembered that the disclosure which follows, together with the accompanying drawings, should not be construed as limiting the teachings of this document to the examples (or embodiments) shown and described. This is so because those skilled in the art to which this disclosure pertains will be able to devise other variations within the ambit of the appended claims.